Vacuum Booster - Partial Brake Assist Diagnostic Tip

Vacuum Booster - Partial Brake Assist Diagnostic Tip

Problem: Poor brake assist - lack of stopping power or having to depress the brake pedal harder than usual to stop.

Solution: If you suspect the brake vacuum booster of providing only partial assist use the following steps to determine if it is a vacuum supply problem or a booster problem.

  1. Pull another vehicle of similar engine size next to the vehicle in question and connect that vehicle's vacuum source to the suspect booster. I usually use an old piece of air line for this purpose.
  2. With the new source vehicle running apply the brake pedal in the suspect vehicle and check for a change in brake assist. If no change is felt then the booster is likely the culprit. If there is a change in the assist (pedal feels normal) then the vacuum plumbing is likely the problem.
  3. If the vacuum plumbing is the problem then closely inspect the components between the booster and the vacuum source to locate the problem.
More info: The vacuum booster has proven to be one of the most reliable parts in the brake system. This reliability has caused many technicians to overlook the booster as a potential source of brake problems. The booster is also high on the list of unpleasant parts to install. With this in mind it is a good idea to make sure the booster is the source of the problem before installing another unit.Unfortunately this article does not allow a full explanation of the inner workings of a booster but you do need a basic idea of what is going on inside to allow accurate diagnosis. The main components inside a booster that control the assist function are the air and vacuum valves. The position of the valves determine what stage the booster is in. There are three stages of booster operation – unapplied pedal (no assist), applied pedal (assist) and hold (isolate assist). When a vehicle is running and the brake pedal is not applied the booster is in a suspended state. The vacuum valve is open and the air valve is closed causing the diaphragm to be “suspended”. When the brake is applied the vacuum valve closes and the air valve opens. Closing the vacuum valve isolates the boost chamber from the vacuum chamber. Opening the air valve allows atmospheric air to enter the boost chamber. The difference in pressures on either side of the diaphragm is what produces the assist. When the driver is at a stop light the pedal pressure is usually reduced causing the booster to go into the hold mode. During the hold mode the air valve closes and the vacuum valve remains closed. This isolates the air in the boost chamber until the driver releases the brake pedal.
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